1. Technical Field
This invention relates to a solid-state imaging device, and specifically to a solid-state imaging device capable of correcting shading, which is caused by angled incident light.
2. Description of Related Art
Nowadays, in such as video cameras and electronic cameras, solid state imaging devices such as CCD image sensors, CMOS image sensors, and amplification type image sensors are widely utilized. Generally, a light receiving section is provided in each pixel of the solid state imaging device with prescribed pitch in length and width, and provided on the periphery of the light receiving section of each pixel is a charge transfer area, in the case of a CCD image sensor, or provided is a wiring area including such as a charge detection amplifier and a gate for signal transfer, in the case of a CMOS image sensor.
FIGS. 10(a) and 10(b) schematically illustrate a structure of a unit pixel in a solid-state imaging device. FIG. 10(a) is a plan view schematically showing unit pixel 3 in the solid-state imaging device, and illustrates a position of photodiode 4 (light receiving section) as a photoelectric transducer in unit pixel 3. Photodiode 4 usually occupies 20-30% of the area of each unit pixel, and the larger the area becomes the higher the sensitivity of the photodiode. In FIG. 10(a), only photodiode 4 and microlens 7 are illustrated, however in actual, such parts as a transistor to read-in signals and a metal wiring are formed in the vicinity of photodiode 4.
FIG. 10(b) is a sectional view of unit pixel 3. As shown in FIG. 10(b), photodiode 4, serving as a photo electric transducer, is embedded in silicone substrate 8, further, above the photodiode, provided are metal wiring layers 5, color filter 6, and microlens 7. In this way, photodiode 4 as the photoelectric transducer, is formed, when viewed from above, under microlens 7, color filter 6, and metal wiring layer 5.
As shown in FIGS. 10(a), 10(b), since it is necessary to arrange a transistor and a wiring, etc., in each unit pixel 3, photodiode 4 is not usually formed at the center of unit pixel 3 (center of photodiode 4 does not coincide with center O′ of unit pixel 3). For example, as shown in FIG. 10(a), photodiode 4 is formed at the area shifted from center O′ of unit pixel 3 in the Y-direction (toward the left side in the figure), and further slightly shifted in the X-direction (downward in the figure), and the transistor and the wiring, etc. are arranged in the other areas.
The solid-state imaging device is structured two-dimensionally by arranging plural unit pixels 3 having this type of structure. In conventional solid-state imaging devices, unit pixels, each having the same structure, are arranged. For example, as shown in FIG. 11, conventional solid state imaging apparatus 10 is structured by arranging unit pixel 3 shown in FIG. 10(a). Therefore, in every unit pixel 3, photodiode 4 is formed at a position shifted in the same direction from center O′ of unit pixel 3. In this conventional case, in every unit pixel 3, photodiode 4 is formed by shifting to left side and further, slightly downward.
In the solid state imaging apparatus having this type of structure, incident light from Z-direction (vertical to the surface of the drawing) enters approximately vertically in the vicinity of center O of the light receiving surface, while in the unit pixel arranged nearer to the periphery of light receiving surface apart from center O, the incident light enters with larger angle. Here, light receiving surface 2 means the surface where incident light enters in the solid-state imaging device, and a whole surface of all unit pixels 3 parallel to the surfaces of the photoelectric transducers is assumed to be light receiving surface 2.
The difference of this incident angle will be described by referring to the sectional view of the unit pixel shown in FIG. 12, which shows the sectional view taken along line B-B′ of FIG. 11. In unit pixel 3, arranged near the center O, the incident light enters approximately vertically to unit pixel 3 as shown by solid lines in FIG. 12. On the other hand, in unit pixel 3 arranged left of line A-A′ of FIG. 11, on the periphery area of light receiving surface 2, the incident light enters at an angle slanted from right to left (from center O of light receiving surface 2 to the periphery) as shown by broken lines in FIG. 12. While, in unit pixel 3 arranged right of line A-A′ of FIG. 11 at the periphery of light receiving surface 2, the incident light enters at an angle slanted from left to right (from center O of light receiving surface 2 to the periphery) as shown by dashed-dotted lines in FIG. 12.
As shown in FIGS. 10(b) and 12, in each unit pixel 3, photodiode 4, which actually perform photoelectric conversion, is not formed at a top surface but is formed below microlens 7, color filter 6, and metal wiring layer 5. Due to this, though there is no problem in cases where the incident light enters vertically to light receiving surface 2, in cases where the incident light enters with an angle, the incident light may be reflected or blocked, by metal wiring layer 5 for example, and the light amount arriving to photodiode 4 may be decreased.
In the pixel structure shown in FIGS. 10(b) and 12, decreased is the receiving light amount by unit pixel 3 arranged at the right of line A-A′ near the periphery of light receiving surface 2. This is because the focusing position of the incident light entering aslant from the left (the light beams indicated by dashed-dotted lines) is deflected out of photodiode 4, and the incident light is reflected or blocked by metal wiring layer 5. While, in unit pixel 3 arranged left of line A-A′ on the periphery area of light receiving surface 2, as shown in FIG. 12, the light beam enters aslant from the right side (the light beams indicated by broken lines), however, since photodiode 4 is formed left in unit pixel 3, the incident light is able to arrives photodiode 4.
Further, in top/bottom direction in FIG. 11, similarly to in the right/left direction, exposure light amount to photodiode 4, in unit pixel 3 arranged at the periphery area of light receiving surface 2 decreases, due to the incident light with an angle.
In this way, since the amount of received light at the periphery area of light receiving surface 2 is decreased compared to that near center O, images at the periphery area become darker and uneven sensitivity (so-called brightness shading) is generated.
FIG. 13(a) shows a graph of the amount of received light in a case where shading is generated. The horizontal axis shows the lateral position of light receiving surface 2, and the vertical axis shows the output value (the amount of received light). In these graphs, the broken line curve indicates the ideal output, and the solid line curve shows the output in the case where the solid-state imaging device relating to the conventional art receives incident light. As shown in this graph, the amount of received light near center O of light receiving surface 2 is large and nearly same as the ideal output. However, nearer the periphery, the amount of received light gradually decreases and goes away from the ideal output value. Thus, in the peripheral area, the amount of received light decreases to darkness. Here, in FIG. 13(a), the amount of received light in the lateral direction is shown, while the amounts of received light at the peripheral area in the top/bottom direction and in the aslant direction similarly decrease to darkness.
Further, the shape of the aperture for photodiode 4 is not necessarily symmetrical, as shown in FIG. 12, for example, there are differences in the amount of received light of photodiode 4 between cases of right slanted incident light and left slanted incident light, therefore, the amount of received light differs in right/left direction or in top/bottom direction on light receiving surface 2. For example in the pixel structure of FIG. 11, the amount of received light in Y-direction (namely the right/left direction) varies as shown by the graph in FIG. 13(b), such that the amount of received light of unit pixel 3 arranged right of center O becomes less than that of unit pixel 3 arranged left of center O. Further, since there are also cases where this difference varies according to the color of the received light, as shown by the graph in FIG. 13(c), slight uneven color (so-called color shading) may be generated in the right/left direction or top/bottom direction.
As a method for correcting such uneven sensitivity (brightness shading), the method in which the pitch of microlens 7 arranged in each unit pixel 3 is made smaller than the pitch of unit pixel 3, and along with going nearer to the periphery of light receiving surface 2, microlens 7 is shifted in the direction to the center of each unit pixel 3 (e.g. Patent Document 1).
Further, a correcting method is known in which a light reflecting section is provided on an inner wall of aperture 9 in unit pixel 3 to correct the uneven sensitivity (brightness shading) (e.g. Patent Document 2).
Patent Document 1: Unexamined Japanese Patent Application Publication No. H01-213079
Patent Document 2: Unexamined Japanese Patent Application Publication No. 2000-198505
According to the method of Patent Document 1, the uneven sensitivity (brightness shading) can be corrected to some extent, however, complete correction of uneven sensitivity (brightness shading) is not possible, and uneven color (color shading) cannot be corrected. Further, according to the method of Patent Document 2, complete correction of uneven sensitivity (brightness shading) is not possible, and similarly to the method of Patent document 1, uneven color (color shading) cannot be corrected.
An object of the present invention is to solve the above-mentioned problems, and to decrease the uneven sensitivity and uneven color by correcting the brightness shading and color shading of the solid-state imaging device.